A. Field of Invention
The present invention relates generally to a cardiac arrhythmia detector in a prosthesis such as an internal or external cardiac defibrillator and pacemaker. More specifically, such a detector comprises a microprocessor used to perform an arrhythmia detection algorithm that detects and analyzes an ECG waveform factor and its irregularity for promptly and accurately discriminating among various types of cardiac arrhythmias, including ventricular fibrillation (VF), ventricular tachycardia (VT), supraventricular tachycardia (SVT), or other arrhythmias.
B. Description of the Prior Art
Sudden cardiac arrest (SCA) accounts for about 76% of sudden non-traumatic deaths in adults and about 50% of all cardiac deaths. Approximately 350,000 Americans experience SCA each year with only about 5% national survival rate. Even in hospital, the percentage of patients who survive SCA is not encouraging. This percentage has remained stable at approximately 15%, and has not improved in the last 30 years. Thus SCA still represents a major and unresolved public health problem.
Ventricular tachyarrhythmia (which includes ventricular fibrillation (VF) and ventricular tachycardia (VT)) is the most common initial incidence of SCA. Unlike other life-threatening conditions such as cancer or AIDS, there is an effective, inexpensive and standard therapy for SCA: timely cardioversion/defibrillation applied by a cardiac stimulator device. Early timely cardioversion/defibrillation (i.e., immediately after onset) is the key to survival, since the chances of success are reduced by 10 percent for every minute of delay of the treatment. Death usually follows unless a normal heart rhythm is restored within 5-7 minutes. Therefore, it is the lack of warning, i.e. detection, and the delay for intervention, not a lack of effective treatment, that accounts for the high death rate following SCA.
The most effective means of saving SCA victims outside a hospital consists of widespread deployment of public access defibrillators as suggested by American Heat Association, and wearable automatic external defibrillators. For diagnosed SCA high-risk patients or SCA survivals, implantable cardioverter defibrillator (ICD) is also an effective treatment. For in-hospital SCA, self-monitoring, self-evaluating, and self-defibrillating monitors, such as fully automatic external defibrillator/monitor Powerheart(copyright)(Cardiac Science, Inc., Irvine, Calif.), and automatic defibrillator module plugged into the existing modular monitoring systems, are the expected effective tools. For both implantable and external automatic defibrillators, the tachyarrhythmia detection algorithm plays the key role for the device""s safety, reliability, effectiveness, ease of use, extent of automatic operations, and widespread acceptance. Prompt and accurate detection of VT and VF is still a major challenge in the defibrillation art. Different tachycardias require different electrical therapies: no electrical therapy needed for the conditions like sinus rhythm, sinus tachycardia (ST), and supraventricular tachycardia (SVT); a comparatively low-energy cardioversion for VT; and a high-energy defibrillation shock for VF. Therefore, the challenge for an effective and successful arrhythmia detector is to discriminate these three types of arrhythmias reliably and accurately. A cardiac device can then treat the appropriate condition on an xe2x80x9cas-neededxe2x80x9d basis. In this way, the false shocks caused by SVT and ST can be avoided, since it causes unnecessary patient distress, and may initiate VT or VF when none previously existed. Moreover, unnecessary treatment applied by an ICD also wastes power.
Differentiating VT from VF makes it allow the treatment of tachyarrhythmia with the lowest energy levels, least painful electrical stimulation pulses, and potentially the most effective therapies. For implantable devices where power source energy and patient tolerance to repeated cardioversion/defibrillation shocks are both limited, therefore, discrimination among these three types of arrhythmias is necessary and important.
Among the methods most widely used for detection of VT and VF in antitachycardia devices is heart rate (HR), and the rate of change of rate or suddenness of onset of tachycardias. Rate stability and sustained high rate also are suggested as additional criteria. Rate and rate-related measures are not a reliable criterion because of difficulty in separating SVT, VT, and VF, due to the overlap of the heart rate for these arrhythmias and the likelihood of missing an R-wave trigger (i.e., ECG dropout) during VF with rapidly changing peak amplitudes.
Another known criterion, the probability density function which was used as the original ICD detection scheme to measure of time the signal is away from the isoelectric baseline, is being gradually abandoned due to its lack of specificity for tachyarrhythmia discrimination.
Along with rate, shape differentiation between ventricular electrograms during sinus rhythm (SR) and VT and VF is another known criteria that can be expected to provide an accurate discrimination using a morphology-based algorithm with correlation analysis and template matching. However, its shortcoming is the necessity of waveform alignment, which is critical to a proper point-by-point comparison. If the test and template signals are not aligned correctly, the result of the waveform comparison can be erroneous. Moreover, aligning the test and template signals and the calculation programs can be a burdensome and time-consuming problem, especially for implantable cardioverter/defibrillator. Furthermore, more memory is required for storing the test and template signals. Therefore, there is still some difficulties for real-time implementation in defibrillators, especially for ICD.
A method of discriminating among cardiac rhythms of supraventricular and ventricular origin by exploiting the differences in their underlying nonlinear dynamics reflected in the morphology of the waveform is disclosed in U.S. Pat. No. 5,645,070, issued to Turcott. A two-channel scatter diagram analysis algorithm for distinguishing VT from VF is disclosed in U.S. Pat. No. 5,404,880, issued to Throne. The shortcoming for these methods is still the computationally complex and more memory requirement. Other algorithms for tachyarrhythmia discrimination utilizing statistical methods were also proposed (Thakor et al., Ventricular Tachycardia And Fibrillation Detection By A Sequential Hypothesis Testing Algorithm, IEEE Trans. Biomed. Eng., 1990, 37:837-843 and Turner et al., Statistical Discriminant Analysis of Arrhythmias Using Intracardiac Electrograms, IEEE Trans. Biomed. Eng., 1993, 40:985-989). However, their effectiveness and practical feasibility still need further investigation.
Modulation domain function (MDF) is effective in discriminating SVT from ventricular tachyarrhythmias (Mattioni et al., Initial Clinical Experience With A Fully Automatic In-hospital External Cardioverter Defibrillator, PACE 1999, 22:1648-1655). However, SVT with an underlying chronic bundle branch block or with aberrant conduction can result in high MDF values, this method may fail for this kind of rhythm. Moreover, MDF cannot differentiate VT from VF.
Currently, AED""s in use are 90% sensitive for ventricular tachyarrhythmia and 90-95% specificity for other heart rhythms. For ICD the percentage of patients who are paced or shocked unnecessarily still exceeds 40% of those receiving ICD therapies. Moreover, discrimination of VT from VF is also a difficulty objective to achieve using existing algorithms. A need still exists for discovering additional information from ECG waveform to develop computationally simple method of discriminating SVT, VT, and VF.
In atrial and ventricular tachyarrhythmias, the shapes of the P-waves and QRS-waves are distorted from the normal sinus rate shapes. Nonshockable arrhythmias have different morphology with shockable arrhythmias (VT and VF). In fact, while physicians classify a cardiac rhythm, they examine the morphology of the ECG in addition to the heart rate. The morphological differences of the cardiac waveform are indicative of cardiac condition changes. One simple and quantitative measurement for the morphological difference is the waveform-factor (WF) disclosed in this invention. Based on WF and waveform-factor irregularity (WFI), one novel cardiac arrhythmia detector is proposed for simultaneously discriminating SVT, VT, and VF.
The new algorithm of present invention as disclosed herein is simple, computationally efficient, effective, and well suited for real-time implementation. Therefore, it offers all the desirable features for the practical application to AED and ICD.
The present invention relates to a cardiac device with a detector that applies waveform factor analysis to physiological signals. An example of the application is to respond to the needs of AED and ICD by providing a cardiac arrhythmia detector, which provides a clearer and more reliable indication of the onset of VT and VF than has been available in the prior art.
It is accordingly an objective of the present invention to provide a cardiac device which can distinguish reliably among VT, VF, and a set of conditions comprising normal sinus rhythm (NSR), sinus tachycardia (ST), and other supraventricular tachycardias (SVT).
A further objective of the present invention is to provide such a detector which is capable of correctly and accurately distinguishing in real time among these three kinds of cardiac episodes using an easy-to-implement algorithm with a minimum amount of computation complexity.
An additional object of the present invention is to provide such a detector which quantifies the nature of VT and VF (higher waveform-factor, WF, compared to SVT) and the nature of VF (higher waveform-factor irregularity, WFI, compared to VT) in order to achieve a diagnosis.
It is still another further object of the present invention to provide such a detector, which discriminates VT from VF and thereby allowing consideration of lower energy therapies for VT to provide significant energy savings for the battery powered device and improved patient comfort, such as an implantable cardioverter/defibrillator, and at the same time avoiding unnecessary shock to SVT.
These and other objects of the invention are realized by providing a novel cardiac device with an arrhythmia detector, which is capable of reliably and efficiently differentiating VT, VF, and nonshockable SVT and atrial fibrillation (AF). Specifically, the detector of the present invention uses ECG waveform-factor and its irregularity for VT, VF, and SVT separation. According to the present invention, ECG under different cardiac rhythms demonstrates different morphology and different comparative change in waveform, which can be characterized by waveform-factor (WF) and waveform-factor irregularity (WFI). For nonshockable tachyarrhythmias (such as SVT and atrial fibrillation), their ECG WF is lower than that of VT and VF. For VT, its WFI is lower than that of VF, since the waveform of VT is more stable and the QRS complexes of VT basically look more similar than that of VF. The detector of the present invention is able to address the limitation of existing algorithms and provide accurate separation among VT, VF, and SVT.
This invention offers a considerable improvement over current methods of analysis by employing a new criterion for reliable separation among VT, VF, and SVT. Particularly, the disclosed WF and WFI measurements quantify the ECG waveform morphology and its change, where lower WF indicates nonshockable SVT and AF, and for shockable tachyarrythmias lower WFI indicates VT, higher WFI indicating VF. In this way, the method determines the precise arrhythmia allowing exact therapeutic selection. Specifically, with proper distinction of SVT, VT, and VF could benefit patients by avoidance of false shock and consideration of lower energy therapies, thereby providing patient comfort and significant energy savings (for ICD). In comparison with known methods of identifying arrhythmias, for the first time the present invention proposes the concepts of ECG WF and WFI and their estimation methods, and for the first time realizes them in a novel arrhythmia detection algorithm.
In one aspect, the present invention provides a cardiac monitor for determining the cardiac condition of a patient, said cardiac monitor including a sensor that senses the intrinsic activity of a patient""s heart and generates a corresponding sensed signal; an interval detector that detects the interval associated with a cardiac cycle based on said sensed signal; a waveform factor detector that detects a waveform factor from said sensed signal, said waveform factor being a function of a mean value of said signal during a cardiac cycle and a peak value of said signal during said interval; and a first comparator that compares said waveform factor to a threshold and generates a corresponding first output indicative of the patient""s cardiac condition.
The monitor may further include a waveform factor irregularity detector that detects a sudden change in said waveform factor and generates a corresponding second output. A second comparator may be used that compares the second output to a second threshold, said second comparator generating a comparator output indicative of one of ventricular tachycardia and ventricular fibrillation based on said comparator output.
In another aspect of the invention, a method of analyzing the cardiac condition of a patient""s heart is provided comprising the steps of sensing a cardiac signal; determining a cardiac interval; determining a waveform factor based on values of said cardiac signal during said cardiac interval and a peak value of said cardiac signal during said interval; comparing said waveform to a first threshold value; and generating respectively a first output indicative of a non-shockable rhythm and a second output indicative of a shockable rhythm. The step of determining a waveform factor may include averaging values of said cardiac signal over said interval and dividing the resulting average by the peak value during said interval to generate an instant waveform factor.
The method may further include averaging said instant waveform factor over several intervals to obtain said waveform factor, and a waveform factor irregularity parameter indicative of a sudden change in said waveform factor. According to the method, a tachyarrhythmia signal is generated if said heart rate exceeds a first threshold and one of a shockable and a nonshockable signal is also generated dependent on a magnitude of said waveform factor, wherein a nonshockable signal is indicative of a supraventricular tachycardia and a shockable signal is indicative of a shockable tachycardia.
In another aspect of the invention, one of a fibrillation and a tachycardia signal is generated dependent on a magnitude of said waveform factor irregularity, said fibrillation signal being indicative of a ventricular fibrillation and said tachycardia signal being indicative of a ventricular tachycardia.
The device and method described can be implemented in either an external or an internal antitachyarrhythmia device.